ProjectID#:Bat369

High Energy Rechargeable Lithium-Metal Cells: Fabrication and

IntegrationJIE XIAO (PI) AND JUN LIU

PACIFIC NORTHWEST NATIONAL LABORATORYJUNE 20TH, 2018

2018 DOE VEHICLE TECHNOLOGIES PROGRAM ANNUAL MERIT REVIEW

This presentation does not contain any proprietary, confidential, or otherwise restricted information

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Overview

• Project start date: December 2016

• Project end date: October 2021• Percent complete: 30%

• Low energy: Li metal anode will boost cell energy

• Short battery Life: mitigating side reactions will extend the cycling stability

• Total project funding: $50M– DOE share: 100%

• Funding for FY 2017: $10M• Funding for FY 2018: $10M

Timeline

Budget

Barriers

• Battery 500 PIs• BNL,ARL,INL,SLAC• 10 universities• GM, Navitas Systems

Partners

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Relevance/Objectives• Overall Objectives

– Overcome the fundamental issues in building high-energy rechargeable Li metal batteries

– Demonstration of long-term cycling of 500 Wh/kg Li metal cells

• Objectives of this period– Identify the cell-level scientific challenges in high-energy rechargeable Li

metal batteries: Li/NMC and Li-S – Demonstrate 300 Wh/kg Li metal pouch cells for at least 50 stable

cycling

• Impacts – Accelerate the development of high-energy rechargeable Li metal

batteries for future vehicle electrification

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Milestones: Keystone Project 3 for Cell Fabrication, Testing and Diagnosis

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Approach• Pouch cell design and analysis to provide a clear roadmap

to build high-energy Li metal cells.

• Identify the key factors that limit the cycling of rechargeable Li metal cells

• New electrolytes and cell optimization to extend reversible cycling of lithium metal anode

• Close the gap between coin cell evaluation and pouch cell testing for fast screening of innovative and effective approaches

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Technical AccomplishmentsDesign and Fabrication of 300 Wh/kg Li/ LiNi0.6Mn0.2Co0.2O2 (NMC622) Pouch Cell

• Appropriate cell designs guide the fabrication of pouch cells with different energy/power and cycling requirements.

• Cathode areal capacity, N/P ratio and electrolyte content are interrelated parameters that dynamically impact the cell energy and cycling stability simultaneously.

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Technical AccomplishmentsLi/NMC622 Pouch Cells Have Very Limited Cycling in Carbonate-based Electrolyte

• Pouch cell capacity always ”dives” after 12 cycles regardless of electrolyte amounts.- NMC622 cathodes still works well in re-assembled cells (backup slide)- Cell is not short as reflected by the voltage profiles after failure (backup slide).

• Refilling electrolyte only partially recovers the capacity. - Electrolyte drying out is NOT the only reason for fast capacity decay.- Li is also being depleted after 15 cycles!

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Technical AccomplishmentsSEI Formation Consumes both Electrolyte and Li

• Electrolyte is continuously consumed to form SEI on newly exposed Li surfaces.

• Part of Li is tightly “wrapped” by insulating SEI layers and loses electronic contact with anode: “dead” Li formation

• ”Dead” Li is continuously accumulated during cycling: Li depletion

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Technical AccomplishmentsLi and Electrolyte Contents are Drastically Different in High- Energy Pouch Cells and Lab-testing Coin Cells

• During each cycle: both electrolyte and Li are irreversibly consumed- Pouch cells: very restricted amounts of Li and electrolyte so the cell degrades

very fast- Coin cells: almost unlimited amounts of Li and electrolyte thus hundreds of stable

cycling is often seen in literature• A standard coin cell testing protocol has been developed to close the gap between

coin cell evaluation and pouch cell fabrication.

1 mAh/cm2

50 mAh/cm2 (250 µm Li)

Typical Configuration in Literature

flooded electrolyte e.g.,75 µL≃ 90 g/Ah(30 times more electrolyte than in pouch cell)

N/P=50

N/P= 2.6

One Repeating Unit

3.81 mAh/cm2

10 mAh/cm2 (50 µm Li)

lean electrolyte i.e., 3.1 g/Ah300 Wh/kg1 Ah

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Technical AccomplishmentsBoth Li and Electrolyte Contents Significantly Impact Cycling Stability of Li Metal Cells

• Cells are terminated once Li or electrolyte (or both) is completely drained, whichever comes first.

• Pouch cells consist of thin Li (50 um) and lean electrolyte (3 g/Ah) • Confirm that the continuous consumption of thin Li and lean electrolyte

leads to fast pouch cell degradation.

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Technical AccomplishmentsNew Electrolyte Mitigates Side Reactions between Li and Electrolyte

• New electrolyte (1.5 M LiFSI-TEP/BTFE) greatly extends pouch cell lifespan (project #Bat362).- Less amount of electrolyte and Li are consumed each cycle- Improved SEI quality to slow down “dead” Li formation- Non-flammable, low viscosity and good wetting

• Independent pouch cell testing at INL demonstrated > 100 stable cycles (project # Bat368)

* J.-G.Zhang et al, Advance Materials, 2018, DOI: 10.1002/adma.201706102

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Technical AccomplishmentsNew Electrolyte Slows Down the Side Reactions between Thin Li and Lean Electrolyte

• In new electrolyte (300 Wh/kg cell):- Cycled Li has larger average particle

sizes: less surface areas for side reactions.

- Li shows less expansion and cell volume expansion is also reduced.

- No uniform external pressure applied yet in both tests.

• Intact Li is found in the cell tested in new electrolyte.

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Technical AccomplishmentsNew Electrolyte is also Compatible with LiNi0.8Mn0.1Co0.1O2 (NMC811) Cathode

• Li/NMC811 chemistry further increases cell energy to > 300 Wh/kg.• Optimized cell design is critical to extend stable cycling to beyond 100 cycles.

Testing conditions: first 2 cycles: C/10 for charge/dischargesubsequent cycles: C/10 charge and C/3 discharge

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Responses to Previous Years Reviewers’ Comments• This project was not reviewed last year.

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Collaboration and Coordination with Other Institutions

• Industry: - General Motors: independent testing of PNNL’s thick S electrodes- Navitas System: new coating method

• University: - UT Austin: Supplied high-Ni NMC to PNNL for evaluation- Univ. Washington: separator coating- SUNY Binghamton: cell fabrication- UC San Diego: testing on industry-made electrodes and PNNL electrolytes- Univ. Pittsburg: supplied S/C composite for electrode coating- Penn State Univ.: testing of thick NMC and S electrodes made- Univ. Houston: testing of PNNL new electrolyte- Stanford: S electrodes testing- Univ. Maryland/Army research Lab: electrolyte development

• National Laboratory- Idaho National Lab: independent testing of PNNL-made pouch cell- Brookhaven Nation Lab: characterization of PNNL fabricated electrodes/electrolytes- SLAC: new electrolyte characterization

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Remaining Challenges and Barriers

• Push the cell energy towards 500 Wh/kg by appropriate cell designs and new electrolytes

• Balance of high energy and cycle life of Li metal cells• Dendrite-induced cell shorting (C/10 charging rate is used

in FY18 to decouple cell shorting and cell failure caused by Li/electrolyte depletion.)

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Proposed Future Work

Any proposed future work is subject to change based on funding levels.

• FY18 (Q4): Demonstrate 350Wh/kg pouch cell with >50 stable cycling and <20% fading

• FY19: - Cell design to balance energy and cycling- Optimization of the new electrolyte to further enhance the

cycling stability of Li/NMC811 pouch cells - Electrolyte and separator modifications to improve Li-S pouch

cell cycling ability- Li metal modification to improve the anode stability

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Summary• ≥300 Wh/kg Li metal pouch cells have been successfully

fabricated and demonstrated >100 stable cycling.• Cell-level fundamentally new challenges are identified.- Pouch cells contain very limited amounts of Li and electrolyte. - The fast depletion of both Li and electrolyte leads to the quick cell

degradation.• New electrolyte has been developed to enhance Li stability

and is also compatible with high-Ni NMC cathodes.• A standard coin cell testing protocol has been developed

and shared across the team to accelerate innovations at relevant scales.

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Acknowledgement

• DOE/EERE/VTO: Battery500 • Key contributors: C. Niu, D. Lv, S. Chen, H.

Lee, X. Ren, S.Q. Wilson, Q. Li, W. Xu and J. Zhang

• Battery500 PIs and their teams